This place is a pawnbroker/loan shark operation. The owner and his guards are members of the Thieves Guild, but his house is not the Guild headquarters (that would be the tavern at 45.) The house acts as a legitimate pawnbroker up front, but the back has a safehouse of sorts. There could perhaps be a hidden tunnel beneath the house that connects to the Thieves Guild HQ at 45 (available to the player only once they have reached a proper rank in the guild, of course)

27. Othesarre: Pawnbroker & Loans

Othesarre (Altmer, m, pawnbroker) Loan shark and Thieves Guild associate. Besides his official shop, Othesarre runs a tidy scam with Guild associates in the Abecette and the harbor dives: people who make gambling debts on the Guild’s rigged tables are directed by friendly "bystanders" to Othesarre, who offers them predatory loans.

Laurianus Tracchus(Colovian, m, rogue) Thieves Guild muscle. Mostly just stands around and looks intimidating, although he sometimes goes out to collect on debtors or do repo work.

Claiming!
I'm not sure about a tunnel though. If #45 was next door that would be a different story, but in this case it seems impracticable; however, I do like the idea of having a private backroom that guild members can hangout in.